Signaling formats and wiring configurations required for enhanced functionality or key type telephone sets (key sets) are incompatible with those required for tip and ring devices (TR devices). Because of this incompatibility, these different set types cannot generally be connected simultaneously to a single station port of a telephone switching unit, e.g., a key telephone control unit. In the context of this application the terms key sets and TR devices are used in accordance with the modern understanding of these terms in the art. Accordingly, key sets are meant to be multifunction telephone station sets or station equipment type adjuncts that utilize special digital messaging protocols to communicate with the control unit concerning actions taken at the set and/or actions that the set should take. TR devices are the well known tip and ring devices including, without limitation: standard, home-use type telephone sets-both rotary and multi-frequency signaling types; answering machines; facsimile (FAX) machines; and modems. As a result of the incompatibility between key sets and TR devices, the customer often must use a more expensive key set where it would be preferential to use an inexpensive TR device, or rely upon special custom adaptors if a less expensive TR device is to be used in conjunction with or instead of a key set.
One commercially available series of telephone systems will accept both key telephone sets and TR devices despite their incompatibility. However, when these otherwise incompatible sets are bridged to a single port in the prior system only the key set is fully functional. On the other hand, the TR device, when connected in such a configuration, cannot be alerted. Consequently, in such a configuration, calls cannot be automatically answered by a conventional answering machine and a facsimile machine bridged with such a key set cannot automatically answer incoming calls. Additionally, in such a configuration, the TR device can not originate calls.